IN his new book, “Racing Weight: How to Get Lean for Peak Performance,” Matt Fitzgerald, a sports nutritionist, writes about an amazing running experience. He worked out on a special sort of anti-gravity treadmill, the AlterG, which uses a cushion of air to lift the body, allowing you to effectively decrease your body weight as you run.

Mr. Fitzgerald started out on the treadmill by running without the machine’s assistance. Then he ran with it adjusted to lift him just enough so that he was 10 percent lighter.

“I felt as if I had become 10 percent fitter,” he writes. Running at his usual pace was suddenly “utterly effortless,” he notes, adding that “it felt like normal running, only so much better.”

Exercise physiologists agree that if your sport is particularly affected by the tug of gravity — running, cross-country skiing, cycling up hills — you are penalized for excess weight. But that leaves some questions: What is the ideal weight for your sport? And how much difference will it make if you actually achieve it?

There have been few direct tests of the body-weight effect, said Hirofumi Tanaka, an exercise physiologist at the University of Texas at Austin. Most of them were done in the 1970s and involved subjects who were asked to run with weights on their backs or ankles. Sure enough, the heavier the people were, the tests showed, the harder they had to work to run at a given speed.

But the runners’ forms were not affected by the extra weight, Dr. Tanaka said. That means that you would probably run the same way if you were heavier. But it would be a lot harder to run at your usual pace, and you’d end up running more slowly.

How much is less clear. Beth Parker, the director of exercise physiology research at Hartford Hospital in Connecticut, said that, for runners, the general rule is that a 1 percent reduction in weight leads to a 1 percent increase in performance.

So, why not just be as thin as you can be?

The problem is that everyone has a point at which further weight loss actually makes their performance worse, said Dr. Mark Tarnopolsky, a muscle metabolism researcher and physiologist at McMaster University in Ontario. Dr. Tarnopolsky, who is a nationally ranked athlete in winter triathlons, adventure racing and ski orienteering, said that people vary so much that there is no formula to figure out the perfect weight.

When Dr. Tarnopolsky was in graduate school, he saw the delicate balance between losing just enough and too much. He and his friends would experiment, losing or gaining a few pounds and testing their VO2 maxes, a measure of the body’s ability to get oxygen to muscles during exercise. In theory, the less you weigh, the higher your VO2 max should be, relative to body weight.

Dr. Tarnopolsky said that he got his best VO2 max — 86 milliliters of oxygen per kilogram of body weight — when he weighed 156 pounds. “Like everyone else, I said, ‘Maybe if I drop some body fat, it will go higher,’ ” Dr. Tarnopolsky said. So he got his weight down to 152 pounds. But to his surprise, his VO2 max decreased, to 82.

The likely reason, he said, was that he had reached a point where his body began burning its own muscle protein for fuel. He was weaker, and his performance was worse, even though he weighed less.

“You could see on the VO2 machine what your body knew was right,” Dr. Tarnopolsky said. “You’d feel tired, stale, lethargic when you tried to drive your weight down.”

Often the only way to know your best weight is by trial and error.

My running coach, Tom Fleming, a former elite runner who won the New York City Marathon twice, in 1973 and 1975, said that he always tells his competitive athletes “that the perfect weight is the weight you are the day you P.B. in your event,” referring to the time you achieve your personal best — or fastest — finish.

“Your body will tell you” your perfect weight, he said, and when you are there, “you will feel fast, race fast.”

Dathan Ritzenhein, an American who is one of the world’s top runners, used a similar system. Mr. Ritzenhein, who broke the national record last year in a 5,000-meter-race and who, at ninth place, was the top American finisher in the marathon at the Beijing Olympics in 2008, said that it took him about 12 years of trial and error to learn his best racing weight. He discovered it last year, his best racing year ever: 121 or 122 pounds (he is 5 feet 8).

That weight is not a natural one for him, he said. If he were to stop training, he would weight about 127 or 128 pounds, and when he is training but not trying to control his weight, he is about 124 or 125. His goal is to try to be at his perfect racing weight a couple of weeks before a big event, losing about a pound a week in the preceding weeks to get there.

He has learned, he said, that if he tries to lose weight too fast or if he continues to lose weight up until his race day, he does not have the energy he needs for his best performance. And if he tries to lose weight too fast, his training suffers.

“It’s a hard line” between losing just enough at the right rate and losing too much too fast, he said.

Other athletes say that they learned through similar experiments.

Andre Agassi, the tennis star, and his longtime trainer, Gil Reyes, discovered through experience that Mr. Agassi’s best weight was between 178 and 182 pounds (Mr. Agassi is 5 feet 11 1/2.)

“We came up with a number, but we did not seek a number,” Mr. Reyes explained in a recent telephone interview. “It was all about him feeling strong and fit.”

Tennis is different from distance running, Mr. Reyes noted. Athletes like Mr. Agassi never know if they will be playing for one or five hours, and they have to be ready for every possibility.

Before he retired from tennis, Mr. Agassi would sometimes gain weight and then stop eating, trying to shed the pounds fast. Mr. Reyes discouraged this. “I said to him, ‘Why do you feel like you have to stop eating to lose that weight?,’ ” he recalled. “ ‘What if you were to eat 10 to 15 percent more, but train 40 percent more?’ ”

Bicyclists in grueling races like the Tour de France also have the problem of maintaining their strength, but for them a little extra weight can make the difference between winning and performing dismally on days when the race has steep hills or mountains.

“I knew from experience and results that I had an ideal weight — or what I thought was ideal,” said Andy Hampsten, a former Tour de France rider and the only American ever to win the Giro D’Italia, in 1988. “If I set too low of a weight goal, I would be weak and stressed,” he said. “If I weighed 4 or 5 pounds more than ideal, I could see I was slower than my competitors.”

Mr. Hampsten, who is 5 feet 9, said that he aimed for a race weight of about 137 pounds, deliberately reducing his intake in the two months before racing season. In the off season he would let his weight drift up to a more comfortable 145 pounds.

The lesson is that, even if the laws of physics and an experience on an AlterG may seem to prove the benefits of a lower weight, exercise science is nowhere near making good predictions for specific athletes, Dr. Tarnopolsky said.

“I know an individual who is one of the fittest ultra-sport athletes,” he said. “She competes in 100 milers, and her body fat is close to 20 percent.”

Yet, he said, “she is one of the most talented athletes I have ever seen.”

Mr. Reyes said that he and Mr. Agassi learned not to let the scale rule your life. “We had a little bit of a phrase,” he said. “The weight scale to most human beings can be like a Ouija board. It can start messing with your head.”